modified label/labelSimulation:
. andbool and orbool are modified so that the true/false constants are

casted to the (integer) type of the enclosing expression, to matchCsem/Cexec?. If the type is not an integer, default on 32 bits.

. labelSimulation proof adapted to match changes.

proof of simulation for expressions Cl to Cm finished
. caveat : eats up all my RAM (8gb) when using matita (not matitac), barely typecheckable
. moved some lemmas from toCminorCorrectness.ma to new file toCminorOps.ma

and frontend_misc.ma to alleviate this, to no avail - more radical splitting required ?

slight modification in SimplifyCasts? to take into account modifications in semantics,
removed some duplicate lemmas and replaced them by wrappers to avoid breaking the
rest of the development.